The U.S. Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11 standard is a family of standards for wireless local area network (WLAN) in 2.4 and 5 GHz bands. The 802.11b standard defines various data rates in the 2.4 GHz band, including data rates of 1, 2, 5.5 and 11 Mbps. The 802.11b standard uses a direct sequence spread spectrum (DSSS) preamble having a period of 1 μs, and modulates data by a clock rate of 11 MHz. In addition to data rates of 802.11b, the 802.11g standard further defines higher data rates of 6, 9, 12, 18, 24, 36, 48 and 54 Mbps in 2.4 GHz band, and uses an orthogonal frequency division multiplexing (OFDM). The preamble includes 10 short periods, each having a period of 0.8 μs, and 2 long periods, and modulates data by a clock rate of 20 MHz. Since both 802.11b and 802.11g may occupy the same frequency band, i.e. 2.4 GHz, to transmit data, it is important for a wireless communication receiver to identify the WLAN standard when receiving data so that it can be demodulated in an appropriate way.
Many wireless communication systems of the prior art include two packet detection circuits to respectively detect the two different WLAN standards. The packet detection circuits are configured to receive the preamble of data to determine the WLAN standard.
FIG. 1 shows a packet detection circuit of the prior art, which is adapted to, for example, detect whether a received data is under 802.11b standard. The packet detection circuit includes a shift register 110, a delay-correlator 130, an auto-correlator 150, an adder 170 and logic 190. The shift register 110 is configured to receive the preamble 100 of the data. Because the packet detection circuit is adapted to detect the 802.11b standard, the shift register 110 is required to store successive 40+1 taps, each of which is at 40 MHz, so that a first storage unit 1101 and a last storage unit 1103 can respectively store two corresponding taps 102, 104 in two adjacent periods. For example, if the tap 102 is the first tap of the fifth short period, then the tap 104 is the first tap of the fourth short period. The delay-correlator 130 is configured to perform delay-correlation to obtain a data moving average of the preamble 100. The auto-correlator 150 is configured to perform auto-correlation to obtain a normalized power moving average of the preamble 100. The adder 170 is configured to compare the normalized power moving average with the data moving average. The logic 190 is configured to determine the result of the comparison. Based on the determination, the wireless communication systems of the prior art are able to identify if the received data is under 802.11b standard or not.
To obtain the data moving average of the preamble 100, the delay-correlator 130 includes seven elements 1301, 1303, . . . , 1313. A first element 1301 executes a complement calculation of the tap 104 and outputs a complement result 106. A second element 1303 multiplies the tap 102 to the complement result 106 and outputs a signal 108 indicating whether the taps 102 and 104 correspond to each other. The correspondence herein means that the sequence of the tap 102 in one period is identical to that of the tap 104 in the immediate preceding period. If yes, the signal 108 asserts HIGH. It happens only when the data is modulated under the 802.11b standard because the number of the storage units of the shift register 110 is particularly decided based on the number of taps in a period under the 802.11b standard. A third element 1305 is another shift register used to store some successive signals 108 in order. The number of the storage units of the element 1305 depends on practical needs. A fourth element 1307 and a fifth element 1309 are respectively an adder. A sixth element 1311 delays the output signal of the fifth element 1309. The elements 1305, 1307, 1309 and 1311 are used to obtain the data moving average 101 of the preamble 100. A seventh element 1313 calculates and outputs an absolute value 112 of the data moving average 101.
To obtain a normalized power moving average of the preamble 100, the auto-correlator 150 includes six elements 1501, 1503, . . . , 1511. A first element 1501 calculates a power value of the tap 102 and outputs a signal 114 indicative of the power value. A second element 1503, a third element 1505, a fourth element 1507 and a fifth element 1509 are used to obtain the power moving average 116 of the preamble 100. Their functions and structures are respectively identical to the third element 1305, the fourth element 1307, the fifth element 1309 and the sixth element 1311 of the delay-correlator 130. A sixth element 1511 is a multiplier configured to normalize the power moving average 116 based on a predetermined factor 118 and to output a signal 120 indicative of the normalized power moving average of the preamble 100.
The wireless communication systems of the prior art need another packet detection circuit as FIG. 1 shows to detect whether the data is modulated under the 802.11g standard. The difference between the two packet detection circuits is the number of the storage units of the shift register 110. The number of the storage units of the shift register 110 for 802.11g/OFDM signals is (32+1) taps instead of (40+1) taps because each period of the preamble of the data modulated under 802.11g/OFDM includes 32 taps.
The drawback of the prior art is the requirement of two packet detection circuits. One can realize that if a wireless communication system is required to identify three different WLAN standards, it needs three packet detection circuits. This increases cost and occupies much IC layout area.